Republican U.S. Senate candidate Michael Sullivan is out with his first television ad of the campaign – a negative spot attacking his Republican opponent, private equity investor Gabriel Gomez.

The ad attacks Gomez for a letter he wrote to Democratic Gov. Deval Patrick asking for the interim appointment as U.S. Senator, in which Gomez said he supported Democratic President Barack Obama in 2008 and supports Obama’s positions on gun control and immigration. Gomez has since said he supported Obama’s opponent, Republican John McCain, and only agrees with some of Obama’s positions.

“The Washington Republican establishment. Will they support a candidate who supporters Barack Obama?” the ad asks, as clips from Gomez’s letter appear on screen. The ad calls Gomez “an Obama Republican.”

The ad then urges voters to vote for Sullivan, an “independent Republican.”

Sullivan spokeswoman Alicia Preston said the ad will air on cable television statewide.

The letter has been a major strike against Gomez for some Republicans, and both of his Republican opponents - Sullivan and State Rep. Daniel Winslow – have raised it in recent debates. However, the ad also illustrates that Sullivan is now taking Gomez, who is new to state politics, seriously as a potential competitor.

Gomez has criticized Sullivan, a former U.S. Attorney, district attorney and state representative, for being a career politician. Sullivan, in a statement, said Gomez is acting like a politician. “What we don't need is another politician in Washington who speaks out of both sides of his mouth and only tells people what he thinks they want to hear," Sullivan said.

“I think it's fair for the voters to compare what Gomez is saying today versus what he privately wrote to Deval Patrick in January, when he practically begged for the job,” Sullivan said.

Gomez responded, in a statement, that Sullivan’s attack is “predictable.” “Career politician Mike Sullivan has launched the first attack ad of this election,” Gomez said. “With 19 days left Mike Sullivan’s campaign is desperate and in full free-fall. Why is he attacking me? Because he is losing…. When will Mr. Sullivan learn that mudslinging and petty politics isn’t going to solve our spending problem in Washington?”

Republican candidatess for U.S. Senate, from left, Gabriel Gomez, Michael Sullivan, and Daniel Winslow participate in a debate sponsored by WBZ-TV and The Boston Globe, Wednesday, April 10, 2013 in Boston. Whoever attracts the most votes will face the winner of the Democratic primary pitting Congressmen Edward Markey and Stephen Lynch against each other. (AP Photo/The Boston Globe, Barry Chin, Pool)The Associated Press